Washington: Hillary Clinton and Donald Trump's ugly and acrimonious battle for the White House is barreling toward the end, with the candidates taking the debate stage tonight for one final primetime showdown.

For Trump, the debate is perhaps his last opportunity to turn around a race that appears to be slipping away from him. His predatory comments about women and a flood of sexual assault accusations have deepened his unpopularity with women and limited his pathways to victory. His supporters remain intensely loyal, but there are few signs he's attracting the new backers he desperately needs.

Clinton takes the stage facing challenges of her own. While the electoral map currently leans in her favour, the Democrat is facing a new round of questions about her authenticity and trustworthiness, concerns that have trailed her throughout the campaign. The hacking of her top campaign adviser's emails revealed a candidate that is averse to apologizing, can strike a different tone in private than in public, and makes some decisions only after painstaking political deliberations.

The last in a trio of presidential debates, tonight's contest in Las Vegas comes just under three weeks from Election Day and with early voting already underway in several key battleground states.

Trump has leaned on an increasingly brazen strategy in the campaign's closing weeks, including peddling charges that the election will be rigged, despite no evidence of widespread voter fraud in US presidential contests. He's also charged that Clinton attacked and intimidated women involved with her husband's affairs, bringing three women who accused former President Bill Clinton of unwanted sexual contact and even rape to sit in the audience for the second debate. The former president has never been charged with crimes related to the encounters, though he did settle a sexual harassment lawsuit.

Trump is bringing President Barack Obama's half-brother, Malik Obama, as his debate guest. Clinton is bringing billionaire and frequent Trump critic Mark Cuban and Hewlett Packard Enterprise CEO Meg Whitman, one of the former secretary of state's highest-profile Republican backers.

Republicans desperately hope Trump can close the campaign by focusing on Clinton's weaknesses, a strategy some privately concede may not be enough at this point for him to win, but could help Republican Senate candidates salvage their races.

The businessman has shown flashes of renewed focus in recent days, including highlighting a senior State Department official's request that the FBI help reduce the classification of an email from Clinton's private server. It was to be part of a bargain that would have allowed the FBI to deploy more agents in foreign countries, though it was not immediately clear whether the State Department official or someone at the FBI first raised that prospect.